double instance of sound when using VSE-scene strip in another scenes VSE
The answer is with AV-sync.

If you have multiple scenes with audio, then make sure that only the one you're currently working on is set to AV-sync.

If you do happen to get a sound instance running that is not apparantly attached to a scene, you can stop it by switching to the scene with AV-sync and start/stop playback once from there, that will stop the "wild" sound instance.

Here's a little script-snippet that will turn off sync on all scenes, then set AV-Sync for this scene only (tested with blender 2.7x - not with 2.8)

import bpy

for scn in bpy.data.scenes:
    scn.sync_mode = 'NONE'`

bpy.context.scene.sync_mode = 'AUDIO_SYNC'